Process for the enameling of articles made of iron with vitreous enamels



Feb. 4, 1964 K EVELBAUER 3,120,458

PROCESS-FOR TI-IE ENAMELING OF ARTICLES MADE OF IRON WITH VITREOUSENAMELS Filed Aug. 25, 1961 SHEET IRON,DEGREASED IN THE CUSTOMARY MANNERIN AN ALKALINE SOLUTION RINSE, COLD WATER ETCHING BATH, 5" 30 MIN. ATROOM TEMP. HYDROCHLORIC ACID= 50% BY VOLUME NITRIC ACID 2 BY VOLUMECHROMIC ACID 0.2% BY VOLUME WATER BALANCE I RINSE, COLD WATER PICKLINGBATH TO FORM THE BLACK LAYER 0,3 I5 MIN. AT ROOM TEMP NITRIC ACID 5% BYVOLUME COPPER SULFATE O.l% BY VOLUME WATER BALANCE RINSE, COLD WATERFIXING BATH,O.5 3 MIN., 50-80C INHIBITED PHOSPHORIC ACID 2 BY VOLUMENICKEL SULFATE 3% BY VOLUME BORAX I% BY VOLUME WATER BALANCE BOILINGWEAKLY-ALKALINE BATH,O,5-IO MIN.

SODIUM NITRITE 0.5 BY VOLUME ENAMELING WITH A WHITE COVERING ENAMEL INTHE CONVENTIONAL MANNER II II II INVENTOR.

KARL EVELBAUER United States The invention relates to a process for theenameling of articles made of iron with vitreous enamels, and, moreparticularly, to the preparation of said iron articles for theapplication of such enamels and to a process whereby a covering coat ofsaid vitreous enamel can be applied without using a prime coat.

The drawing is a flow diagram illustrating an embodiment of theinvention.

For the enameling of iron goods with vitreous enamels, it has hithertobeen the common practice to degrease these iron goods, to etch or picklethem, to wash and to dry them preparatory to spraying them with theenameling mass or applying the same by powdering, allowing these massesto dry, and firing them. As a general rule, the iron goods, such askitchen utensils, etc., are provided with a prime glaze, onto which oneor more cover glazes are laid. However, the prime glaze frequently isomitted, and a covering glaze is applied directly to the surface of theiron vessel, it being feasible to lay on a number of further coveringglazes, if desired. For the direct enameling of the iron surface, it isnecessary to use an enamel'of a particular composition and/or to subjectthe iron to a special surface treatment.

Upon etching or pickling the iron goods, preparatory to enameling in themanner described above, the phenomenon frequently occurs whereby a blacklayer covers a portion or even the entirety of the iron surface.Heretofore, it had been assumed that this black layer, during theensuing enameling process, exhibits disadvantageous actions and/or flawsin the enamel glaze. In particular, thedetrimental occurrence known as aboil-up of the enamel during firing of the glaze has been ascribed tothese black layers. Also, the local scorching of the base or primeenamel upon firing of the iron goods coated with a prime enamel has beenbelieved to be caused by these black layers. Hence, it has always beenendeavored to avoid the formation of such black spots or layers duringetching or pickling of the iron articles preparatory to enameling of thesame. When these black spots or layers did form, the iron goods weresubjected to a special treat ment in order to eifect their removal.

The instant invention is based on the surprising discovery that a blacklayer formation on pickled iron goods, contrary to previous contentions,not only is not detrimental to the glazing of these articles, butactually has a favorable influence upon the enameling process, such as asmooth glazing and an adhesion of the enamel to the iron which isconsiderably stronger than normally attained. Especially, it has beenestablished that iron articles having the black layer on their surfacecan be coated immediately with a covering enamel without first applyinga primer or base enamel.

On the basis of this discovery, the process for the enameling of irongoods according to the invention consists in forming the black layer onthe degreased iron surfaces, said black layer covering these surfaces.The black layer then is fixed on the iron surfaces by immersion in anickel plating bath, whereafter the iron goods are passivated andsubsequently enameled in the conventional manner.

A single etching or picking sufiices to produce the black layers.However, it is of distinct advantage to etch the iron articles in afirst step with a known etching agent in aqueous solution, such ashydrochloric acid, nitric acid, or chromic acid, to rinse themthereafter in water, and subsequently to pickle them as a second step,in order to produce the black layer, in an oxidizing pickling bath, suchas an aqueous solution of nitric acid and copper sulfate. In allinstances, it is important that the black layer thus formed iscontinuous and uninterrupted.

To the step wherein the black layer is produced according to theinvention, a fixing step, for instance in a nickel plating bath, is tobe added, followed by passivation. As the nickel plating bath,opportunely an aqueous solution of phosphoric acid, nickel sulfate, andborax is used, whereas the passivation can be carried out, in a mannerknown per se, in an aqueous solution of sodium nitrite and borax, towhich advantageously phosphates may be added. It is preferred topassivate the goods treated without rinsing after the fixing step,whereas a rinsing with water is hardly avoidable subsequent to thepickling steps.

The first etching step and the pickling step wherein the black layer isproduced advantageously are carried out cold, i.e. at room temperature.The fixing step opportunely is effected at approximately 5080 C.,whereas it is recommended to passivate in a boiling bath. The firstetching step, depending upon the bath concentration, advantageously iscarried out within approximately 5-30 minutes, the second pickling step,also in dependence on the bath concentration, within approximately 0.3to 15 minutes, and the fixing step within 0.5 to 3 minutes.

Albeit the process according to the invention is designed primarily fora single-layer enameling of alloyed or unalloyed sheet iron, especiallyfor the application of white glazes, it goes without saying that it isapplicable to any and all other kinds of enameling of iron goods.

The process according to the invention now will be further illustratedby the following example. However, it should be' understood that this isgiven merely by way of explanation, not of limitation, and that numerouschanges may be made in the details without departing from the spirit andthe scope of the invention as hereinafter claimed.

I Example A cold-rolled sheet iron, having the following analysis wasused:

This sheet iron was degreased in the customary manner in an alkalinesolution which may contain detergents and emulsifiers, as commerciallyavailable for degreasing of metal articles, It was then rinsed with coldwater. The

degreased sheet was subjected to the first etching step withinapproximately 5 to 30 minutes at room temperature. The etching bath hadthe following composition:

Percent by volume Hydrochloric acid 50 Nitric acid 2 Chromic acid 0.2Water Balance The sheet thus etched was rinsed with water and thenimmersed in the pickling bath to form the black layer, as the secondstep, at room temperature for approximately 0.3 to 15 minutes, wherebythe black 'layer formed. The bath had the following composition:

Percent by volume Nitric acid 5 Copper sulfate 0.1 Water Balance Percentby volume Inhibited phosphoric acid 2 Nickel sulfate 3 Borax 1 WaterBalance After that, the sheet, without rinsing, was passivated in aboiling weakly-alkaline bath for approximately 0.5 to minutes. The bathwas an aqueous solution containing a mixture of sodium nitrite (0.5percent by volume), and borax (0.5 percent by volume), to which, ifdesired, phosphates could be added in the customary manner.

The sheet iron thus provided with a fixed continuous black surface layerwas then coated with a White covering enamel in the conventional manner,and then was fired, also conventionally.

What is claimed is:

1. In a process for the enameling of degreased iron articles withvitreous enamel, the steps comprising producing a pickling residue inthe form of a continuous black layer on the surfaces of said articles bypickling in an oxidizing solution substantially at room temperature forapproximately 0.3 to minutes, fixing said black layer in a nickelplating bath at temperatures of approximately 50-80 C. for approximately0.5 to 3 minutes, passivating them in a boiling, weakly alkaline,solution for approximately 0.5 to 10 minutes, and enameling the articlesthus treated.

2. A process for the enameling of degreased iron articles with a singlecovering vitreous enamel glaze, obviating the need for applying a primeenamel coat, which comprises producing a pickling residue in the form ofa continuous black layer on the surface of said articles by pickling inan oxidizing solution substantially at room temperature forapproximately 0.3 to 15 minutes, lfixing said black layer in a nickelplating bath at approximately 50- 80 C. for approximately 0.5 to 3minutes, passivating the same in a boiling, Weakly-alkaline, solutionfor approximately 0.5 to 10 minutes, applying a covering vitreous enamel{glaze to the articles thus treated, and firing said glaze.

3. In a process for the enameling of degreased iron articles withvitreous enamel, the steps which comprise etching said articles in anacid bath substantially at room temperature for approximately 5 tominutes, rinsing them with water, pickling them in an oxidizing solutionsubstantially at room temperature for approximately 0.3 to 15 minutes,thus forming a pickling residue in the form of a continuous black layeron the surfaces of said iron ar i l i ing aid articles with water,fixing said black layer in a nickel plating bath at a temperature ofapproximately 50-80 C. for approximately 0.5 to 3 minutes, passivatingsaid articles in a weakly alkaline, boiling bath, for approximately 0.5to 10 minutes, and then enameling the articles thus treated.

4. A process for the enameling of degreased iron articles with a singlecovering vitreous enamel glaze, obviating the need for applying a primeenamel coat, which comprises etching said articles in an acid bathsubstantially at room temperature for approximately 5 to 30 minutes,rinsing them with water, pickling them in an oxidizing solutionsubstantially at room temperature for approximately 0.3 to 15 minutes,thus forming a pickling residue in the form of a continuous black layeron the surfaces of said articles, rinsing said articles with water,fixing said black layer in a nickel plating bath for approximately 0.5to 3 minutes at approximately 50-80 C., passivating said articles in aweakly-alkaline boiling bath for approximately 0.5 to 10 minutes,applying a covering vitreous enamel glaze to said articles thus treated,and firing said glaze.

5. In a process for the enameling of degreased iron articles withvitreous enamel, the steps which comprise etching said articlessubstantially at room temperature for approximately 5 to 30 minutes in abath consisting substantially of Percent by volume Nitric acid 2Hydrochloric acid 5O Chromic acid 0.2 Water Balance rinsing acidarticles with water, pickling them, substantially at room temperature,for approximately 0.3 to 15 minutes in a bath consisting substantiallyof Percent by volume Nitric acid 5 Copper sulfate 0.1 Water Balance thusforming a pickling residue in the form of a continuous black layer onthe surfaces of said articles, rinsing the same with water, fixing saidblack layer by immersing said articles at a temperature of approximately50-80 C. for approximately 0.5 to 3 minutes, in a bath consistingsubstantially of Percent by volume Inhibited phosphoric acid 2 Nickelsulfate 2 Borax 3 Water Balance passivating said articles immediatelyafter the fixing operation in a boiling bath for approximately 0.5-l0minutes, consisting essentially of Percent by volume Sodium nitrite 0.5Borax 0.5 Water Balance Percent by volume Nitric acid 2 Hydrochloricacid 50 Chromic acid 0.2 Water Balance rinsing said articles with water,pickling them substantially at room temperature for approximately 0.3 to15 minutes in a bath consisting essentially of Percent by volume Nitricacid 5 Copper sulfate 0.1 Water Balance Percent by volume Inhibitedphosphoric acid 2 Nickel sulfate 2 Borax 3 Water Balance passivatingsaid articles immediately after the fixing operation in a boiling bathfor approximately 0.5 to 10 minutes, consisting essentially of Percentby volume Sodium nitrite 0.5 Borax 0.5 Water Balance applying a coveringvitroeous enamel glaze to said articles thus treated, and firing saidglaze,

References Cited in the file of this patent UNITED STATES PATENTS1,779,273 Hommel Oct. 21, 1930 2,772,183 Talmey et a1. Nov. 27, 19562,817,610 Newell et al Dec. 24, 1957 2,961,337 Bryant et al. Nov. 22,1960

1. IN A PROCESS FOR THE ENAMELING OF DEGREASED IRON ARTICLES WITHVITREOUS ENAMEL, THE STEPS COMPRISING PRODUCING A PICKLING RESIDUE INTHE FORM OF A CONTINUOUS BLACK LAYER ON THE SURFACES OF SAID ARTICLES BYPICKLING IN AN OXIDIZING SOLUTION SUBSTANTIALLY AT ROOM TEMPERATURE FORAPPROXIMATELY 0.3 TO 15 MINUTES, FIXING SAID BLACK LAYER IN A NICKELPLATING BATH AT TEMPERATURES OF APPROXIMATELY 50-80*C. FOR APPROXIMATELY0.5 TO 3 MINUTES, PASSIVATING THEM IN A BOILING, WEAKLY ALKALINE,SOLUTION FOR APPROXIMATELY 0.5 TO 10 MINUTES, AND ENAMELING THE ARTICLESTHUS TREATED.